A cell in a radio communication system is traditionally defined as the coverage area of the system broadcast channel. In most 3rd Generation Partnership Project (3GPP) radio communication systems, such as Global System for Mobile communication (GSM), High Speed Packet Access (HSPA), and Long Term Evolution (LTE), the coverage area of the data channels is identical to the coverage area of the broadcast channel. However, as the performance of current systems is improved with novel features supporting Coordinated Multi-Point transmission and reception (COMP), multi-carrier, and multi-hop, the coupling between system information coverage and data coverage becomes increasingly unmotivated. As will be described below there are several problems associated with this coupling.
Support for Self Optimizing Networks is Limited
If we want to automatically optimize system parameters that affect the coverage, e.g. antenna tilt, antenna beam-width, antenna pointing direction, base station transmission power, etc. then we will always risk that coverage is lost in some locations in the network. The only way we can observe this coverage loss is if customers call in and report to their operator that their service no longer works. The operators typically do not accept this and therefore, antenna and power parameters are planned when the system is deployed and are then left unchanged. The fear of breaking something that works is a show-stopper for many automatic network optimization algorithms. As a consequence most systems operate with sub-optimal settings, which is a problem.
Energy is Wasted in the Network
Now with the introduction of LTE Release-10, we have designed a system capable of achieving 1 Gbps data throughput. Future LTE releases are expected to provide even higher bitrates and even better system capacity. While there is a commercial drive for higher data transmission rates and higher capacity, there is no need at all to transmit more system information because of that. When we need to increase the capacity in an area then we need to add more cells and as a result the system information channels become over-dimensioned. If an area already has sufficient coverage for system information channels then there is no need for the new cells in that area to transmit any system information at all. In current systems, where every antenna and every carrier in the network has to be observable all the time, most of the energy consumed in the network is spent transmitting system overhead.
Support for Advanced Antenna Techniques Becomes Limited
Traditional beam-forming, where the antenna radiation pattern is adjusted towards a single UE, does not work if that implies that the coverage area of the broadcast channel is affected. In LTE Rel-8 user specific beam-forming is supported to some extent by means of using different pre-coding weights for data and broadcast signals. However we can not adjust the individual antenna elements such that energy is concentrated towards where a user is located since that would also affect the coverage of the broadcast channel.
High Interference Also During Low Load Limits Performance
Since each cell need to continuously transmit system information and mobility measurement signals (i.e. the primary common pilot channel, P-CPICH, in WCDMA and cell specific reference symbols, CRS, in LTE) we will always have a minimum amount of interference in the system. This non-traffic data related source of interference, sometimes known as pilot pollution, significantly limit the system performance during times of low traffic. Without pilot pollution the peak rates, especially during low traffic hours, would be significantly increased.
In order for the mobile stations to perform coherent demodulation of downlink data they need a reference signal as a demodulation reference. The reference signal must be transmitted in an identical way as the data to be demodulated. Hence in case we want to transmit broadcast information in a large area covering several network nodes then we must also transmit the demodulation reference signal over exactly the same large area.
In systems where the BCH is cell specific each cell also has its own demodulation reference signal and this problem does not occur. The idle UEs camps on one cell in a location area and listens to paging messages from that cell. The paging message is demodulated by using the same cell specific demodulation reference signals that is used for demodulation the BCH transmission.
However, when there is little or no relation between the coverage of a BCH Area defined as the coverage area of the system broadcast channel and the coverage area that can be provided by a single network node then we will have problems with basic system functions such as paging and random access responses.
The paging problem can be described in the following. The system wants to send a paging message to a mobile station and we assume that the network has some a-priori information on the location of the mobile station within the BCH area. Assume that the network knows, or has reason to believe, that the mobile station is in the coverage area of RBS1 or RBS2. Then there is no need to transmit the paging message in the whole BCH area. That would limit the possible size of the BCH area to a rather small number of participating nodes, it would require more physical resources to be spent on transmitting paging messages, and it would reduce the possibilities for cell DTX since there will be paging messages to transmit much more often in any particular node.
There is thus a need to overcome the prior art disadvantages.